1. Field of the Invention
The invention relates to a method for measuring misalignment, and more particularly, to a method to measuring misalignment for exposure with an arc alignment mark.
2. Description of the Related Art
In the fabrication process of an integrated circuit, typically, a wafer is patterned by first transferring a pattern on a photomask to a photoresist layer via an exposure step. As the integration of the integrated circuit continuously increases, the alignment precision for the exposure step becomes more and more crucial.
In the conventional exposure step, an exposure alignment mark is formed on the wafer. While exposing the photoresist layer, a certain patterned is aligned with the exposure alignment mark, so as to precisely transfer the patterned from the photomask to a required position on the wafer. However, this kind of alignment method has error due to resolution limitation. When the fabrication process of integrated circuit approaches a submicron stage, the error is obvious to affect the quality of the wafer.
To avoid the alignment error to affect the product yield during the exposure step, currently, an exposure step is performed on a wafer among a batch of wafers (normally 25 wafers), followed by measuring misalignment on different positions of the wafer. A modification for exposure is then calculated on the other wafers in the same batch.
FIG. 1A shows a top view of a conventional structure to measuring misalignment for exposure.
In FIG. 1A, a wafer 100 comprises exposure alignment marks 102 and an outer layer mark 104. The outer layer mark 104 is arranged as a rectangle as shown in the figure. The outer layer mark 104 by formed by four trenches 104a, 104b, 104c and 104d. The wafer 100 further comprises an inner layer mark 106 made of photoresist. The inner layer mark 106 is formed by forming a photoresist layer on the wafer 100 first, followed by exposure and development steps using a certain pattern on the exposure alignment marks to confirm the position of the wafer 100. As a result, the inner layer mark 106 has a rectangular shape and arranged within the rectangle of the outer layer mark 104. Similarly, the inner mark 106 comprises four strip structures 106a, 106b, 106c and 106d.
The inner layer mark 106 is predetermined to be formed in the center of the outer layer mark 104. Each of the strip structures 106a to 106d is supposed to be equally distant to a corresponding trench of the trenches 104a to 104b. However, as misalignment occurs for exposure, the inner layer mark 106 deviates the central position of the outer layer mark 104. The level of the deviation indicates the magnitude of misalignment for the wafer 100 beyond this region.
To introduce the conventional inspecting method of the misalignment in more details, in the following description, the horizontal direction is denoted as an X-axis, while the vertical direction is denoted as Y-direction as shown in FIG. 1A. A first measuring line 108 is selected parallel to the X-axis. The first measuring line intersects with the trenches 104a and 104c at A.sub.1 and A.sub.4, and intersects with the strip structures 106a and 106c at A.sub.2 and A.sub.3, respectively. The X-coordinates for A.sub.1, A.sub.2, A.sub.3 and A.sub.4 are X.sub.1, X.sub.2, X.sub.3 and X.sub.4. A difference between a mean value of X.sub.1 and X.sub.4 and a mean value of X.sub.2 and X.sub.3 represents misalignment along the X-axis.
A second measuring line 110 is selected to be parallel to Y-axis. The same method is used to obtain a misalignment along the Y-axis. The misalignment as a form of displacement can thus be obtained and modified.
However, while a circular misalignment occurs as shown in FIG. 1B, each side of the inner layer mark 106 and a corresponding side of the outer layer mark 104 meets at an angle. The parallel relationship is not sustained any more. Therefore, using the above method, the misalignment is variable according to the location of the measuring lines. For example, in FIG. 1B, the misalignments obtained by measuring lines 112 and 114 are in different magnitude. This is the same for the measuring lines in vertical Y-direction. Thus, the calculated modification for exposure of the wafers is not correct. The incorrect modification thus causes the pattern to be transferred from the photomask to the wafers with a distortion.
On the other hand, before performing the exposure on the photoresist layer over the wafer, deposition steps are very often performed on the wafer. Some depositions are thus remained within the trenches of 104. Though the trenches 104 are not filled thereby, in case that the deposition is not formed uniformly or is formed with defects, it would cause the outer layer mark 104 out of position to further cause the system fails to inspect the misalignment for exposure.